With demand for high-efficiency energy-saving technologies increasing in recent years, MOSFETs are required to have smaller size, higher breakdown voltage, lower on-resistance, and lower capacitance. To meet these requirements, technologies of embedding field plate electrodes made of polysilicon in trench structures are attracting attention. For example, a Tunable Oxide Bypass U-MOSFET (TOBUMOS) can provide low on-resistance while having high breakdown voltage.
To use polysilicon in a trench structure as an electrode, the polysilicon layer needs to be doped with a high concentration of impurities. Phosphorus atoms are therefore introduced into the polysilicon layer by thermal treatment in an atmosphere of H3POCl. However, this treatment generally causes the phosphorus atoms to be excessively introduced into the polysilicon layer, resulting in volume expansion. The volume expansion applies a high stress to layers adjacent to the field plate electrode, causing dislocations or the like in those layers. Further, the volume expansion causes a top surface of the field plate electrode to protrude or bulge, which prevents the area of contact between an upper layer thereof and the field plate electrode from being consistent.